This invention relates generally to instruments for injecting or withdrawing fluids. More particularly this invention relates to syringes having means for sensing pressure on either injection or withdrawal of fluids.
Syringes of numerous types are used in the medical field for injecting and withdrawing fluids from cavities and vessels in the body, for subcutaneous injection, and for other purposes. The fundamental design of such syringes has not changed significantly over the years. Almost all such syringes have a hollow barrel, a plunger which is inserted into the barrel from one end, and a hollow tip at the other end of the barrel for receiving a cannula, a tube or other device for conducting fluid into or out of the syringe.
In many applications fluids must be injected or withdrawn without creating excessive positive or negative pressure. For example, the act of introducing fluids into a vessel or into a balloon that is positioned in a vessel or duct lumen transmits pressure to the vessel or duct wall. These transmitted forces can be damaging to the vessel or duct wall, causing architectural alterations and even occasionally wall disruption. It is critical that undue pressure be avoided when injecting fluid into small vessels in the body, particularly small vessels of infants, children and adults. Similarly, excessive pressure must be avoided in hydrostatic dilatation of free or in situ vein graphs for venous or arterial bypass procedures, and in the inflation of endotracheal, urinary, biliary, vascular and other balloon catheters.
The provision of means for sensing pressure applied on either injection or withdrawal of fluids by syringe would make it far easier to avoid the application of undue pressure. The provision of such pressure sensing means would have other important medical applications such as in procedures where the lumbar spinal canal must be penetrated by syringe while the cerebral spinal fluid pressure is monitored.
Embolectomy catheters, used to remove blood clots from blood vessels, comprise a new type of catheter into which fluid must be injected to inflate a balloon, where the need for avoiding the application of excessive pressure is acute. Such catheters are inserted through an incision into a blood vessel and moved with the catheter balloon deflated to a point beyond the clot, whereupon the balloon is inflated and the catheter is withdrawn pushing the clot ahead of it until it reaches the incision where it can be readily withdrawn. If excessive transmitted wall pressures and excessive wear stresses are applied to the vessel wall as the balloon is inflated or withdrawn, the vessel can be seriously damaged or even ruptured. If the balloon is severely over-inflated, it can, in rare instances, fragment leaving portions of the catheter in the vessel and creating the danger of obstructions in the vascular system.
U.S. Pat. No. 4,444,188 to the present inventors describes in one embodiment an embolectomy catheter in which the balloon is inflated by applying finger pressure directly to a resilient chamber integral with the catheter and in direct communication with the balloon. Squeezing the chamber inflates the balloon with air and thus the chamber provides the user with direct, tactile information regarding the fluid pressure of the inflated balloon, so that the user can react to excessive balloon pressure by relaxing the finger grip. While the approach of the '188 patent has found widespread acceptance, the provision of a syringe for filling the catheter with either air or liquid could enhance the operation of this device. Unfortunately, a conventional syringe would be woefully inadequate in this or any other application in which the user must quickly and accurately sense the gas or liquid pressure being applied by the syringe because in a conventional syringe the pressure signal is masked by the friction between the plunger and the inside wall of the barrel.